In a typical automotive vehicle driveline, it is common practice to include a hydrokinetic torque converter. The impeller of the torque converter receives engine torque and the turbine of the torque converter transfers torque to the torque input element of multiple-ratio gearing of the transmission.
The presence of the torque converter in the torque flow path introduces hydrokinetic power losses, particularly during startup of the vehicle, as the torque converter fluid in the converter torus circuit is accelerated and decelerated. These losses are manifested in heat loss to the hydrokinetic fluid, which requires a heat exchanger to maintain an acceptable hydrokinetic fluid temperature.
Attempts have been made to eliminate the power losses inherent in a powertrain having an internal combustion engine and a torque converter automatic transmission by replacing the engine with an electric motor, the power output element of the motor being connected to the torque input element of the transmission. Such driveline arrangements, however, do not take advantage of the superior performance of an internal combustion engine in an automotive vehicle. Further, they require the presence of an on-board electric voltage source.
Attempts to combine the advantages of an internal combustion engine with an electric motor drive have been made in hybrid vehicle arrangements, but the engine is required in such known designs to operate throughout a large speed range including startup speeds and to operate at idle speed while the vehicle is at rest.